1. Field of the Invention
The present invention relates to a photo-sensor cell suitable for use as a photo-sensor for detecting rectilinear or angular position or displacement when a number of photo-sensor cells are integrated into a single chip.
2. Description of the Prior Art
FIG. 1 shows a typical example of prior-art photo-sensor circuits, which comprises a light sensing element and a signal processing circuit integrated into a single chip, as disclosed in "SHARP TECHNICAL REPORT" No. 26, pages 127 to 130, June 1982. In FIG. 1, each photo-sensor circuit comprises a photodiode 1, an amplifier 2 connected to the photodiode 1, a Schmitt trigger circuit 3 triggered by an output signal of the amplifier 2 so as to provide a hysteresis, an output transistor 4 for amplifying an output signal of the Schmitt trigger circuit 3, a load resistor 5 connected to a collector of the output transistor 4, and a supply voltage stabilizer 6 for supplying a stabilized supply voltage to the photo-sensor circuit. Here, the Schmitt circuit outputs a high-voltage level output signal (e.g. "1") when an input signal voltage rises beyond a predetermined DC level (e.g. determined by an emitter resistor) and a low-voltage level output signal (e.g. "0") when the input signal voltage drops below the same DC level so as to provide hysteresis due to the presence of a turn-on voltage (e.g. base-emitter voltage) of a switching element(e.g. a transistor).
Therefore, when weak light is allowed to be incident upon the reversely-biased photodiode 1, although a small optical current flows through the photodiode 1, the Schmitt trigger circuit 3 is not turned on. However, when a strong light is allowed to be incident upon the photodiode 1, since a relatively large optical current flows through the photodiode 1, the Schmitt trigger circuit 3 is turned on, so that it is possible to obtain an output signal of "0" in response to the intensity of light allowed to be incident upon the photodiode 1. In the prior-art photo-sensor circuit, since a digital output signal can be obtained by comparing the incident light intensity (transduced into an input voltage) with a reference voltage, it is possible to eliminate unstable operation due to noise, for instance.
In the prior-art photo-sensor circuit, however, since the light current detecting section is made up of an amplifier and a Schmitt trigger circuit, when a number of circuits are integrated into a single chip, there exists a problem in that the number of elements is large and therefore the area occupied by the chip increases. In addition, since a detected incident light is compared with a reference voltage after having been transduced from light into electricity through the photodiode and the amplifier, there exist other problems in that when the reference voltage fluctuates due to temperature difference, in particular, the light sensitivity of the photo-sensor cell is unstable because of temperature dependency. The above-mentioned temperature-dependent unstability is serious, in particular when a number of series-connected photodiode and Schmitt circuits increases in a multichannel mode.